LAST year, Chris Smith saw 25 stars explode. He is a lucky man. In the three millennia before the 20th century, we recorded far fewer supernovae. Now these stellar cataclysms are being collected like new species of beetles: over the next decade, astronomers should spot thousands. But this catalogue of stellar disasters is only a means to an end. In a project called ESSENCE, based at the Cerro Tololo Inter-American Observatory in Chile, Smith and his colleagues are hunting something far more important than explosions. They are on the trail of arguably the deepest mystery in physics: dark energy.
This cosmic puzzle was first encountered in 1998, when two groups of astronomers reported that dozens of distant supernovae appeared surprisingly faint (麻豆传媒, 11 April 1998, p 26). They concluded that the expansion of the Universe must be accelerating, dimming these distant explosions. It came as rather a shock, because although we have known for more than 70 years that the Universe is expanding, people had always assumed that the expansion must be slowing down. The gravity of all the matter in the Universe should be putting the brakes on the expansion.
But the supernovae showed that about 10 billion years ago something began to overpower gravity, making the expansion accelerate. It鈥檚 as if gravity were working in reverse, or as if a cosmic poltergeist were grabbing galaxies and flinging them outwards.
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So far, we know almost nothing about dark energy. But that鈥檚 about to change. Astronomers like Smith are beginning to try and pin down its nature. The plan is to trace exactly how the expansion is speeding up, and therefore just how repulsive dark energy is. 鈥淚t is like trying to figure out how many cylinders a car engine has by watching the car accelerate,鈥� says ESSENCE team member Peter Garnavich from the University of Notre Dame in Indiana. 鈥淭welve cylinders are going to get the car zooming better than four. We are watching the Universe start to accelerate after the matter-dominated era. How fast it takes off is a clue to what is driving it.鈥�
The acceleration of the Universe is too gentle to feel, so instead astronomers detect it using a particular kind of stellar explosion called a type Ia supernova. These come from binary star systems where a small white dwarf, the ember of a star like our Sun, is orbiting a red giant. If the white dwarf is close enough, it pulls material off the red giant until eventually it reaches a critical mass and begins to collapse. That heats up the white dwarf so much that the carbon and oxygen nuclei in it suddenly fuse, releasing enough heat to blast the star apart.
Because these stars all have the same critical mass, and therefore the same amount of nuclear fuel, the explosions are all of similar brightness. So if astronomers compare that known value with their apparent brightness as seen from Earth, they can work out how much the light has spread out during its journey, and therefore how long ago the star went bang.
They then look at the red shift of the supernova light. As the light travels through an expanding Universe, its wavelength gets stretched, as if the light wave were drawn on the surface of an expanding balloon. The increase in wavelength moves the light towards the red end of the spectrum. So by measuring the wavelength of light from, say, hydrogen atoms in a supernova and comparing it with the light from hydrogen in the laboratory, astronomers can work out the red shift, and therefore find out how much cosmic expansion took place during its journey to Earth.
Measure enough supernovae at different red shifts, and astronomers can tell whether the light has travelled through space that is expanding at a constant rate, or ever more slowly, or ever faster. Because red shift tells you how much space has stretched, and brightness tells you how much time has elapsed, you can plot the size of space against time 鈥� the history of the cosmos reduced to a line on a graph (see 鈥淪tretching space鈥�). This was how teams led by Saul Perlmutter of the Lawrence Berkeley Laboratory in California and Brian Schmidt of Mount Stromlo Siding Springs Observatories originally discovered the acceleration: the graph turned up instead of down. By looking even more closely at how the graph takes off, astronomers might just identify the culprit behind dark energy. Or that is the plan: the suspects are still rather shadowy figures.
At the moment we have only one clue. Einstein鈥檚 general theory of relativity, which describes gravity as a distortion of space and time, says that gravitational forces are generated not only by mass and energy, but also by pressure. If you have stuff under high pressure, that adds to the gravity it generates. Conversely, if you have stuff under tension, it produces negative, repulsive gravity. Pick up a rubber band or a spring and stretch it, and you have just set up an antigravity field.
But it is not up to much. In fact, the positive gravity produced by the rubber band鈥檚 mass will overwhelm the tiny antigravity created by its tension. No ordinary substance can be stretched hard enough to produce a net antigravity effect. So what kind of extraordinary substance could do it?
Educated guesswork
Physicists have made several guesses. The first thing they thought of was empty space 鈥� or rather, the not-quite empty space of the quantum vacuum, where fluctuations in energy allow short-lived virtual particles to constantly appear and disappear. Forces between these particles could give space-time a tension. Unfortunately, when you calculate how much antigravity the vacuum ought to have, using quantum theory plus a bit of guesswork, you get a number 10123 times too big. With a vacuum energy like that, every molecule in the Universe would explode. 鈥淚f that鈥檚 your leading candidate,鈥� says Michael Turner of the University of Chicago, 鈥渂oy, you haven鈥檛 made much progress.鈥�
This vacuum energy (sometimes called the cosmological constant) also leaves us with a puzzling coincidence: we seem to be living at a special time, when the vacuum energy is roughly equal to the energy density of matter. The latter is decreasing all the time, but the vacuum energy is a constant. When it was set, at the time of the big bang, it would have been many orders of magnitude smaller than the matter energy density; in the distant future, it will be many orders of magnitude bigger. Why should the two values be about the same now?
Uncomfortable with the cosmological constant, physicists have devised other tense substances. 鈥淨uintessence鈥�, for example, was developed in 1998 by three cosmologists, Robert Caldwell at Dartmouth College in New Hampshire, Paul Steinhardt of Princeton University and Rahul Dave at the University of Pennsylvania. They imagined a kind of dark energy a bit like a thin fluid filling space. Its energy density can change over time and it has always been relatively close to that of matter, so the coincidence is not such a problem.
But there are many versions of quintessence, all with different properties. And it is not clear what the stuff actually is.
Even stranger is the idea of 鈥渢opological defects鈥�. These are mismatches in space-time left behind by critical events in the Universe鈥檚 history 鈥� when the so-called electroweak force split into separate weak and electromagnetic forces, for example. Topological defects would be like giant walls, stretched across the Universe and under high tension.
Or maybe there is no such thing as dark energy, it is just that we鈥檝e got gravity wrong. Perhaps, over very large distances, the force of gravity emanating from ordinary matter changes from attractive to repulsive. Possibly, some kind of new theory of gravity could even account for both dark energy and dark matter 鈥� that other nagging problem, of the Universe鈥檚 missing mass.
It is all guesswork, though. Any of these things might be true 鈥� or none of them. 鈥淚 strongly suspect that the correct answer is not on this list,鈥� says Turner.
But there is only one way to find out: pin down the expansion history of the Universe and see if it matches any of the theories. Cosmologists label the repulsiveness of dark energy with a single number, w, the ratio of pressure to energy density. Quantum vacuum energy has a w of -1, so the stuff has a lot of tension, and it is very repulsive. In some versions of quintessence, w is -0.5, which is only fairly repulsive (see 鈥淐osmic antigravity鈥�). The value of w corresponds to the number of cylinders in Garnavich鈥檚 car, so measuring how fast the acceleration kicks in could tell us what kind of dark energy surrounds us. But the effect is pretty slight, so in order to tell what is really going on we need a lot of supernova data.
Testing times
Smith and his team use the Blanco 4-metre telescope at Cerro Tololo. In October, November and December of last year, they spent every other night searching the same small patch of sky, sitting up to analyse the images. 鈥淚t was pretty gruelling,鈥� Smith says. They were looking to see if any of the galaxies in that patch of sky had suddenly acquired a bright spot 鈥� a candidate supernova. Follow-up observations with other telescopes checked the kind of explosion and measured the red shifts of the 15 newly discovered type Ia supernovae, giving 15 new points on the expansion graph.
But that still does not pin down the graph tightly enough, because there are slight uncertainties in the measurements. We know that type Ia supernovae are not all exactly the same brightness, for example, and although most of the variation can be corrected for by observing other characteristics of the explosions, a small uncertainty remains.
Later this year, ESSENCE should collect a few dozen more supernovae. And so on for another five years. It will take at least three more years and a couple of hundred supernovae before the data are good enough to really narrow down the possible range of w. By the end of the project, Smith hopes to get it to within 10 per cent of its true value.
That will certainly narrow down the possibilities. Topological defects would have a w of 鈥�2/3, so ESSENCE will either rule them out, or, more excitingly, rule out vacuum energy. 鈥淚f we show that w is not -1, it means the dark energy is something even weirder than quantum vacuum energy,鈥� says Turner.
But either way, ESSENCE will not rule out all the models 鈥� there are too many of them. Some versions of quintessence say that w changes, and that it should have homed in close to -1 during the past few billion years. That would make quintessence look very like vacuum energy, and the only way to tell them apart would be to trace the expansion back 10 billion years 鈥� to when there was last a significant difference between them. You cannot do this with the ground-based telescopes that ESSENCE uses because the light from these very distant supernovae has been stretched to 2.5 times its original length, red-shifted right out of the visible band into the infrared. At these wavelengths, the atmosphere itself is so bright it messes up the measurement.
To see that far back in time, you need to go into space. And that is just what the Supernova/Acceleration Probe (SNAP) will do. As it orbits the Earth, it will search for these ancient explosions in a patch of sky 鈥� actually, 20 separate patches of sky, each about five to six times the size of the full Moon. The aim is to catch about 6000 supernovae, and monitor their light as they each brighten and fade over a period of a few weeks.
If the plan is approved by Congress at the end of this year, says Perlmutter, SNAP could launch as soon as 2009. Within three more years it could pin down w to within 5 per cent, and pick up a change as small as 15 per cent over the 10 billion years. It is an impressive prospect: SNAP could finally provide the clues needed to work out the nature of dark energy.
That does not mean we will understand the source of dark energy straight away, though. 鈥淗ere鈥檚 one doomsday scenario,鈥� Turner says. 鈥淲e build SNAP and find that w is -0.88, and that it varies with time 鈥� but we still don鈥檛 understand it for a hundred years.鈥�
It is a fair point. Other puzzling aspects of the cosmos, such as dark matter, have resisted explanation for many decades, even though we have known more about them than we do about dark energy. But Perlmutter is more optimistic. By plotting the past expansion in fine detail, he hopes we can do more than just weed out some of the rival models. Perhaps it will indicate a new explanation of dark energy, one that fits in neatly with other mysteries of cosmology, such as the nature of inflation, or even dark matter. 鈥淚 hope we鈥檒l get that 鈥榓ha!鈥� moment and see a fundamental unifying principle that will make sense,鈥� he says.
Even if there is no such epiphany, measuring the change in w could at least tell us the distant future of the Universe. If the value of w is fixed, then dark energy will always be repulsive. It will keep accelerating the Universe outwards, stretching it into bland emptiness. But if it can change, we might be in for a more interesting future. In February, Renata Kallosh and Andrei Linde of Stanford University pointed out that dark energy could eventually move from being repulsive to being attractive (Journal of Cosmology and Astroparticle Physics, DOI: 10.1088/1475-7516/2003/02/002). Then the Universe would stop accelerating, put on the brakes, and collapse inwards. Time would end in a big crunch.
The supernova data already gathered shows that no such turnaround is happening just yet. According to Kallosh and Linde鈥檚 calculations, that means a collapse is unlikely to happen within the next 18 billion years. But the far more detailed measurements from ESSENCE and SNAP should let us look very much further ahead, picking up advance warning of a big crunch a trillion years from now. Even Turner might agree that, given so much time, we could figure out what dark energy is.